This invention was conceived in the course of the execution of a grant from the National Heart and Lung Institute. With the exception of nuclear powered pumps and standard roller-pumps, both clinically and experimentally used blood pumps are, at the present time, routinely energized from a pneumatic power source. The pumps may be biventricular, intended for total heart replacement, or, they consist of a single ventricle intended for use as an auxiliary pump assisting the natural heart, usually the left side of the heart. If these pumps are to function optimally, there is a need for precise, instantaneous measurement of blood volume within the pump. Such a monitoring system should provide (1) direct, on-line monitoring of dynamic pump behavior, including magnitude of displacement and rates of filling and ejection, and (2) the means for closed loop, automatic control of pump operation. The need for such a monitoring system is particularly great if a pump is implanted inside the body as this excludes visual observations. These needs have not previously been met.
The measurement of a varying capacitance between two plates is a common method for measuring displacement, for instance, in a displacement or in a pressure transducer. The principle is applicable to pneumatic blood pumps, as described in Vol. XII, No. 1, pages 3-12, Cardiovascular Research Center Bulletin, 1974, since they contain, within a relatively rigid housing, two compartments separated by a flexible diaphragm or bladder wall. Since one compartment contains blood and the other gas, such as air, expansion of one compartment at the expense of the other will, conceptually, change the electrical capacitance across the pump, the dielectric constant of blood being much greater than that of air or gas. The device here described utilizes this fact. In the practical application to pneumatic blood pumps, it has been found expedient and advantageous to monitor not the capacitance across the pump, but rather the capacitance across the gas (air) space as this varies reciprocally with instantaneous blood volume within the pump. The capacitance to be measured ranges from a fraction of a picofarad (pf) in some pumps to a maximum of 30-40 pf in other pumps.
Theoretically, the simplest way to measure the relatively small electrical pump capacitances is to attach appropriate electronic circuitry directly to or inside the pump. In practice, this arrangement has several drawbacks, the most important being the problem of long-term reliability and electrical stability. The problem is compounded if the pump is implanted inside the body: (1) body fluids represent a hostile environment for electronic circuitry; (2) space is frequently at a premium; (3) electronics are inaccessible; the replacement of a sense module would require additional surgery; (4) if telemetry is utilized, implantation of a power source creates additional space, encapsulation, and duration problems.
The following specifications and requirements of a pneumatic blood pump monitoring system are incorporated in the present invention: (1) all active electronic components and circuitry can be located at a convenient distance (12 feet or more) from the pump; (2) it can easily be adapted to various pump configurations; (3) it can be employed with implanted pumps without requiring separate transcutaneous (across-the-skin) cable channel; (4) it electronically provides (a) on-line direct readout of the instantaneous blood volume within the pump; (b) output which can be displayed in analog form on oscilloscope or standard recorder; (c) output which can be utilized in closed loop, beat-to-beat, automatic control of pump operation; (d) good signal-to-noise ratio; (e) long-term stability; (f) high immunity to interference.